1. Field of the Invention
The present invention relates to an electric double layer capacitor comprising polarizable electrodes immersed in an organic electrolyte, and a method for producing the same.
2. Description of the Related Art
The conventional electric double layer capacitor, which is based on the use of an organic electrolyte, uses so-called activated carbon having a specific surface area of not less than 1000 m2/g, as an active material for its polarizable electrodes.
The electrostatic capacity is expressed in the electric double layer capacitor as follows. That is, the expression is achieved by a treatment which is called xe2x80x9cactivationxe2x80x9d to artificially form pores in a carbon material, the pores being controlled so that the solute of the organic electrolyte may be adsorbed (see, for example, Japanese Laid-Open Patent Publication No. 9-320906).
However, in the case of the conventional electric double layer capacitor, the electrostatic capacity and the withstand voltage depend on the activation method, and they have had the following limits. That is, in the case of the steam activation method, the electrostatic capacity has had the limit of about 15 F/cc and the withstand voltage has had the limit of about 3 V. In the case of the alkaline activation method, the electrostatic capacity has had the limit of about 20 F/cc and the withstand voltage has had the limit of about 2.5 V.
The present invention has been made taking the foregoing problem into consideration, an object of which is to provide an electric double layer capacitor which makes it possible to achieve an electrostatic capacity and a withstand voltage exceeding those of the conventional electric double layer capacitor based on the activated carbon system, in the case of the use as a polarizable electrode of the electric double layer capacitor, and a method for producing the same.
According to the present invention, there is provided an electric double layer capacitor comprising polarizable electrodes mainly composed of a carbon material immersed in an organic electrolyte, wherein the carbon material has graphite-like microcrystalline carbon, and an electrostatic capacity is expressed by insertion of ion of a solute of the organic electrolyte into a space between microcrystalline carbon layers of the carbon material.
According to the present invention, it is possible to achieve an electrostatic capacity and a withstand voltage which exceed those of the conventional electric double layer capacitor based on the activated carbon system.
In the present invention, it is preferable that the carbon material is a carbon material having the graphite-like microcrystalline carbon produced by performing a heat treatment at not less than a temperature at which alkali metal vapor is generated, together with at least one of an alkali metal and an alkali metal compound.
Those preferably used as the solute include tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of quaternary ammonium; tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of tetraethylammonium (TEA+) or tetrabutylammonium (TBA+); tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of triethylmethylammonium (TEMA+); tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of quaternary phosphonium; and tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of tetraethylphosphonium (TEP+); as well as tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of imidazolium derivative represented by the following general formula: 
wherein R1 and R2 are alkyl groups each having 1 to 5 carbon atoms, and R1 and R2 may be either the same group or different groups; and tetrafluoroborate (BF4xe2x88x92) or hexafluorophosphate (PF6xe2x88x92) of 1-ethyl-3-methylimidazolium (EMI+).
Those usable as a solvent of the electrolyte without any problem in the electric double layer capacitor according to the present invention in which the solute ion is inserted into the space between the graphite-like microcrystalline carbon layers contain at least one of propylene carbonate (PC), xcex3-butyrolactone (GBL), ethylene carbonate (EC), and sulfolane (SL) which are standard solvents for the conventional electric double layer capacitor based on the activated carbon system.
It is also allowable to use a solvent composed of a mixture comprising a primary solvent containing at least one of propylene carbonate (PC), xcex3-butyrolactone (GBL), ethylene carbonate (EC), and sulfolane (SL); and a secondary solvent containing at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC).
In the case of the conventional electric double layer capacitor based on the activated carbon system, the secondary solvent has been added in order to lower the viscosity of the electrolyte and improve the electric conductivity. Also in the case of the electric double layer capacitor according to the present invention in which the solute ion is inserted into the interlayer of the graphite-like microcrystalline carbon, equivalent effects have been observed concerning, for example, reduction of internal resistance.
The secondary solvent may be defined such that if the substance is used singly, the performance as an electrolyte solvent is low, however, it is possible to obtain the performance equivalent or superior to that obtained by using the primary solvent or the secondary solvent alone, by combining the substance with the primary solvent. There is no special limitation for the amount of addition of the secondary solvent, for example, not more than 50%.
According to another aspect of the present invention, there is provided a method for producing an electric double layer capacitor comprising polarizable electrodes immersed in an organic electrolyte, wherein a carbon material for forming the polarizable electrodes is prepared by effecting a first heat treatment step of carbonizing a raw material to allow graphite-like microcrystalline carbon to grow, and a second heat treatment step of performing a heat treatment at not less than a temperature at which alkali metal vapor is generated.
According to the present invention, it is easy to obtain the electric double layer capacitor which makes it possible to achieve the electrostatic capacity and the withstand voltage exceeding those of the conventional electric double layer capacitor based on the activated carbon system.
The raw material, which is usable in the present invention, includes, for example, those based on petroleum (petroleum pitch (tar) and petroleum coke), those based on coal (coal pitch (tar) and coal coke), those based on plant (wood material such as coconut husk and sawdust), and those based on resin (phenol resin, polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), and polyimide). However, there is no limitation to the raw materials described above.
According to still another aspect of the present invention, there is provided a method for producing an electric double layer capacitor comprising polarizable electrodes immersed in an organic electrolyte, wherein a carbon material for forming the polarizable electrodes is a carbon material including graphite-like microcrystalline carbon produced by effecting a heat treatment together with at least one of an alkali metal and an alkali metal compound at not less than a temperature at which alkali metal vapor is generated; and a voltage of not less than a rated voltage is firstly applied between the polarizable electrodes after assembling the electric double layer capacitor so that ions of a solute of the organic electrolyte are forcibly inserted into a space between microcrystalline carbon layers composed of the carbon material to express an electrostatic capacity.
According to the present invention, it is easy to obtain the electric double layer capacitor which makes it possible to achieve the electrostatic capacity and the withstand voltage exceeding those of the conventional electric double layer capacitor based on the activated carbon system.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.